You could proxy the images -
https://vborg.vbsupport.ru/showthread.php?t=288060 (I didn't like this myself).
Tbh as most servers now are https enabled and most modern browsers support it the easy way is just to add a upgrade-insecure-requests header and the browser will grab the https version, this is all I do on my sites and never have any problems.
Add to your headerincludes template
Code:
<meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">